Macaques inoculated with doses of SIV below the threshold required for seroconversion and recovery of virus exhibited T-cell proliferation in response to SIV envelope synthetic peptides. Some macaques previously exposed intravenously to subinfectious doses of SIV were challenged 16 months later with an intrarectal dose of SIV. Naive macaques (never exposed to SIV) became infected, but some of the previously exposed animals resisted the virus challenge. The inability to infect macaques exposed to a subinfectious dose of SIV suggests that a T-cell immunity may confer long-term protection against infection and raises the possibility that AIDS vaccines should be designed to optimize the cellular arm of the immune system. The initial study was performed with a small number of macaques, and the results need to be extended to a larger number of animals. In the current study, M. nemestrina will be exposed to subinfectious doses of SIV. After a cell-mediated response is generated in the absence of a humoral response, they will be challenged with an infectious dose of SIV, either intravenously or intrarectally. In 1997, preliminary blood tests, including the development of cell lines for CTL (cytotoxic T cell) assay, were continued.